In detecting the illumination angle of a conventional total internal reflection microscope, the total reflection angle has been calculated and secured by detecting the position of the illumination light in the entrance pupil of the object lens via a CCD or a position sensitive detector (PSD) (for example, see Patent Document 1). In this case, the positional precision of the illumination light depends on the resolution of the detection element and the projection magnification of the incident pupil of the objective lens 16 relative to the detection element, and in order to improve precision, an expensive high-resolution detection element must be used, or the projection magnification must be increased and a larger detection optical system must be permitted. If the projection magnification is increased, the size of the detection element must also be made larger, leading to cost increase. Describing specifically, the position detection resolution of a general CCD or PSD is on the order of several μm to several tens of μm, and this multiplied by the reciprocal of the projection magnification of the objective lens from the incident pupil to the detection element corresponds to the collecting position detection precision of the illumination light in the incident pupil. That is, the higher the projection magnification is, the higher the position detection precision can be made.